If you only have a obd lm317(hope that's the correct name) interface, you can do lot of more things with ibus.
if you have an ediabas interface, you can do more things as ibus. but it is difficult to inplement the software to a carpc front end or android.

e38 and e39 have nearly the same hardware. when they have the same production year.
i think ibus is the best solution.

I didn't know that about the iBus and controlling it, and I'll definitely work on implementing it for v2 of my build. (Mostly because there's an iBus interface by Resler that is backordered for a while and I'm not keen on the available apps working it, I'll let them mature a bit more). Using the Joycon, however, will give me all the functionality I desire (and then some) with hopefully fewer headaches. I'm hoping to get it up and running in a month -- and, if all goes according to my plan, there might be quite a few more E38 owners snooping about here!

(To my knowledge only a handful of DSP E38s have been done, and none as extensive AND OEM looking as what I'm planning!)